1. Field of the Invention
The present invention relates to a technique which allows conducting a doping process or other chemical and physical treatments efficiently even at a low temperature.
2. Prior Art
Known processes for doping semiconductors with impurities include a diffusion process and an ion implantation process. The diffusion process comprises heating the semiconductor to a high temperature in the range of from 1000 to 1200° C. to make the impurities diffuse into semiconductors. In an ion implantation process, a predetermined portion of a semiconductor is bombarded with an ionized impurity which has been accelerated in an electric field.
The diffusion coefficient D of an impurity can be expressed with an exponential function of absolute temperature T as D=D0·exp[−Ea/kT], where D0 is the diffusion coefficient at T=∞, Ea is the activation energy, and k is the Boltzmann constant. This equation describes the increase of diffusion coefficient with elevating temperature; accordingly, it has been common practice to carry out diffusion at temperatures as high as possible, preferably, at 1000° C. or higher. In the ion implantation process, on the other hand, it is necessary to activate the impurity and to remove the defects in the crystal lattice damaged by the ion bombardment; i.e., the implantation is followed by high-temperature annealing in the temperature range of from 600 to 950° C.
Recently, some types of active-matrix liquid crystal display devices using a thin-film transistor (TFT) provided on a glass substrate as the switching device have brought into practical use. The source and drain regions in the TFTs of those display devices are, in general, formed monolithically with the ohmic contacts using amorphous silicon having either of the N-type and P-type conductivity. Because the TFT used in this case is of an inverse stagger type, it likely produces a parasitic capacitance ascribed to its structure. To prevent this unwanted capacitance from developing, there has been made studies on making use of a TFT having its source and drain being formed in a self-aligned structure. However, the source and drain can be formed in a self-aligned manner only by the use of an ion implantation or ion shower process. Then again, a post annealing at the temperature range of from 600 to 950° C. should be carried out to activate the impurities and to recover the damage. Taking into consideration that the general purpose economical glass resists only up to a temperature of about 600 to 700° C., those ion implantation and ion shower processes are not feasible in an industrial operation.
As another means to circumvent the problem concerning the recover of thermal damage on the glass substrates, there is known a technology, i.e., impurity doping using a laser beam irradiation. There is known, for example, a process which comprises first covering the intended portion of the surface of the semiconductor with a thin film of the impurity, and then irradiating a laser beam thereto to melt the thin film of the impurity simultaneously with the surface of the semiconductor. In this manner, it is possible to dissolve the impurity into the surface of the molten semiconductor.
In the process above using an excimer laser beam irradiation, the impurity doping can be carried out without causing thermal damage on the glass substrate. However, the process requires an additional step of coating the semiconductor with the impurity. Conventionally, a coating process such as spin coating has been used for this step. However, the quality of this coating is process-determining, because the concentration of the doped impurity depends on the evenness of this coating. Thus, this process is far from being an ideal one. Furthermore, this coating is formed generally using an organic solvent as the solution medium. The use of such an organic solvent sometimes allows unfavorable elements such as carbon, oxygen, and nitrogen to enter into the semiconductor to impair the properties thereof.
In the light of the circumstances described above, the present invention has been achieved with an aim to provide a laser-beam doping technology using particularly an excimer laser, said technology being composed of simplified process steps and free from invasion of foreign elements into the semiconductor during the process. Accordingly, the present invention provides, with an object to simplify the process and to prevent inclusion of undesirable elements, a doping process using a high purity doping material in its gas phase in the place of the conventional solid or liquid phase doping materials. It is another object of the present invention to increase the doping efficiency.
Still other objects of the present invention include doping of elements into, in addition to semiconductors, various types of materials inclusive of insulators and conductors, as well as modifying materials and surfaces thereof. There can be specifically mentioned, for example, doping of phosphorus into a silicon oxide film.